Ion beam implanters are used to implant or "dope" silicon wafers with impurities to produce n or p type extrinsic materials. The n and p type extrinsic materials are utilized in the production of semiconductor integrated circuits. As its name implies, the ion beam implanter dopes the silicon wafers with a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers. If p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium will be implanted.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion beam implanter includes beam forming and shaping structure extending between the ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
Ion beam implanters have recently been proposed for use in fabricating flat panel displays. Flat panel displays are frequently used in portable personal computers. The displays of such computers do not have a cathode ray tube for displaying text and graphics. Instead, a glass substrate covered with an amorphous silicon solid supports an electrode array for activating discrete picture elements (pixels) of the display. During fabrication the glass is covered with a resistive pattern and then inserted into an implantation chamber so that the ion beam from the source can treat the flat display. This use of an ion implanter requires a larger cross section ion beam to implant an entire width of the flat panel display.
For existing high current ion implanters, the wafers at the implantation station are mounted on a surface of a rotating support. As the support rotates, the wafers pass through the ion beam. Ions traveling along the beam path collide with and are implanted in the rotating wafers. A robotic arm withdraws wafers to be treated from a wafer cassette and positions the wafers on the wafer support surface. After treatment, the robotic arm removes the wafers from the wafer support surface and redeposits the treated wafers in the wafer cassette. In the proposed use of an ion implanter for flat panel displays, the panels are mounted to a support that positions the panel within an extended area ion beam formed by multiple exit apertures in a source.
Operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to surfaces of the implanter beam forming and shaping structure adjacent the ion beam path and also on the surface of the wafer support facing the ion beam. Contaminant materials include undesirable species of ions generated in the ion source, that is, ions having the wrong atomic mass.
Another source of contaminant materials results from operating the implanter to implant different species of ions in consecutive implants. It is common practice to use the same implanter for implants utilizing different ions. For example, the implanter may be utilized to implant a quantity of wafers with boron ions having an AMU of 11 (atomic mass units). The boron implant may be followed by an implant of arsenic ions having an AMU of 75. Such consecutive implants with different ion species may lead to contamination of the second implant wafers with ions from the first implant. This is referred to as "cross specie contamination."
Another contaminant is photoresist material. Photoresist material is coated on the wafer surfaces prior to ion beam treatment of the wafer and is required to define circuitry on the completed integrated circuit. As ions strike the wafer surface, particles of the photoresist coating are dislodged from the wafer and settle on the wafer support surface or adjacent interior surfaces of the beam forming and shaping structure.
Over time, the contaminant materials build up on the beam forming and shaping structure and the wafer support surface and decrease the efficiency of the ion beam implanter and the quality of the treated wafers. As the contaminant materials build up on the implanter component surfaces, upper layers of contaminant materials flake off or are dislodged by ions which strike the contaminant materials, creating discharges and contaminating the implantation of the wafers. Some of the dislodged contaminant material moves along the beam path to the implantation station and is implanted in the wafers. Such contaminant material changes the electrical properties of the wafers. Even a small amount of contaminant material may render the implanted wafers unsuitable for their intended purpose in the manufacture of integrated circuits.
Additionally, buildup of contaminant materials on the interior surfaces of the ion implanter will reduce the efficiency of certain beam forming and shaping components. For example, the ion beam passes through an ion beam neutralization apparatus which partially neutralizes the positively charged ion beam such that the implanted wafers are not charged by the beam. The ion beam neutralization apparatus produces secondary electron emissions to partially neutralize the positively charged ions as they pass through the apparatus. A build up of contaminant materials on the interior surfaces of the ion beam neutralization apparatus impedes the secondary electron emission process.
The contaminants deposited on the implanter interior surfaces must be periodically removed. Removing contaminant materials from the beam forming and shaping structure and the wafer support requires disassembly of the ion beam implanter. The contaminated components are removed from the implanter and carried to a cleaning station since certain dopant materials are toxic. Component surfaces are scrubbed with solvents and abrasives to remove the contaminant materials. The implanter is then reassembled and tested prior to resuming wafer treatment.
This cleaning procedure represents a significant economic cost in terms of implanter down time. In addition to the time required for cleaning the components, reassembly of the implanter is a slow process. Precise alignment of the implanter components must be achieved for proper operation of the implanter. Additionally, the vacuum in the interior region of the implanter must be reestablished prior to operation. Finally, it is standard operating procedure not to allow a production run on an implanter that has been disassembled until it is requalified by implanting test wafers and evaluating the wafers.